Immunology I

• Immunology I covers the topic of innate defenses.

Immune System Overview

• Provides resistance to disease-causing microorganisms.
• Functions as a system rather than an organ.
• Involves molecules and immune cells in lymphoid tissue and blood circulation.
• Distinguishes between innate and adaptive immunity.

Overview of Defense Lines

• The body has three main lines of defense, similar to a medieval castle.
• The first line of defense prevents invaders from entering, like the walls and moat of a castle.
• The second line attacks anything not recognized as "friend", like castle guards.
• The third line attacks only known enemies and has memory, like an army of soldiers.

Innate Defenses

• Surface barriers like skin and mucous membranes.
• Internal defenses include phagocytes, natural killer cells, inflammation, antimicrobial proteins, and fever.

Surface Barriers

• Skin and epithelial/mucous membranes act as physical barriers to prevent pathogen entry.
• The skin is multilayered.
• Keratin stops bacterial enzymes and toxins.
• Produce protective chemicals that inhibit bacterial growth.
• Acidity in skin and stomach inhibits bacterial growth, known as the acid mantle.
• Lysozyme enzymes found in saliva, respiratory mucus, and lacrimal eye fluid destroy bacteria.
• Mucin in respiratory and digestive tract mucus traps microorganisms.

Internal Defenses: Phagocytes

• Neutrophils and macrophages are types of phagocytes.
• Macrophages are "big-eaters."
• Free macrophages wander and are derived from WBC monocytes.
• Fixed macrophages reside in tissues and arise early in development.
• Pathogens have sugar molecules on their surface that phagocyte receptors recognize and bind to.

Phagocytosis Steps

• Adherence: Phagocyte adheres to pathogens or debris.
• Engulf: Phagocyte forms pseudopods to engulf particles, creating a phagosome.
• Lysosome Fusion: Lysosome fuses with the phagocytic vesicle, forming a phagolysosome.
• Digestion: Toxic compounds and lysosomal enzymes destroy pathogens.
• Exocytosis: Indigestible and residual material is removed.

Natural Killer (NK) Cells

• Non-specific killers like virus-infected cells and cancer cells.
• Identify infected/cancerous cells by detecting lack of "self" cell-surface proteins called MHC class I.
• MHC class I is the major histocompatibility complex.
• Attack target cells by releasing perforins and granzymes, causing membrane damage.
• Nuclear disintegration and inflammation is also induced.

Internal Defenses: Inflammation

• Nonspecific response to any localized tissue injury.
• Inflammation signs include redness, heat, swelling, and pain.
• The inflammatory process begins with the release of chemical 'alarm' signals.
• A 'chemical alarm' is released by injured/stressed tissue cells and immune cells (histamine, kinins, prostaglandins).
• Small blood vessels dilate (redness, heat).
• Capillary permeability increases (leakier) causing fluid to cause swelling and pain.

How Inflammation Aids Healing

• Limits movement.
• Increases metabolic rate of cells.
• Delivers oxygen and nutrients.
• Delivers clotting proteins to interstitial fluid.
• The above all help prevent the spread of damaging agents and promote healing.

Neutrophil Mobilization

• Neutrophil mobilization starts soon after inflammation begins.
• Leukocytosis occurs, where injured cells release leukocytosis-inducing factors to increase WBC formation.
• Margination is where neutrophils cling to the capillary wall.
• Diapedesis is the process of neutrophils exiting blood vessels.
• Chemotaxis is what is called when neutrophils follow chemical trails to the injury site.

Internal Defenses: Antimicrobial Proteins (Interferons)

• Viruses cannot replicate themselves because they lack ATP-generating or protein-synthesizing machinery.
• Virus-infected cells can secrete antimicrobial proteins called interferons.
• A virus enters a cell, and substances strip off the virus's outer protein coat.
• Viral nucleic acid gets into the cell’s chemical manufacturing system.
• Cells switch to making new viruses and may be destroyed in the process.
• The new viruses are released to infect other cells.

Steps of Interferon Action

• Virus enters host and replicates.
• Interferon genes are switched on in infected cells.
• Infected cells secrete interferons.
• Interferons stimulate the production of antiviral proteins in uninfected cells.
• Antiviral proteins prevent virus replication.

Internal Defenses: Fever

• Abnormally high body temperature, a systemic response to invading microorganisms.
• Bacteria cause leukocytes to travel to the hypothalamus, which increases body temperature (called pyrogens).
• Benefits include increased ability of immune cells to migrate to lymph nodes.
• Liver & spleen sequester iron and zinc, limiting bacterial access.
• Increased metabolic rate of tissue cells accelerates repair.